High performance ceramics are finding their way more and more into automobile applications. Advanced ceramics have high compressive strength and hardness combined with useful strength. On the other hand, however, ceramics are brittle. Ceramics fail catastrophically, are flaw sensitive, and are difficult to prepare reproducibly. All these characteristics must be taken into consideration for any potential application.
Automotive applications often require tribological surfaces with minimal lubrication or high temperature performance. Specific strength and modulus are particularly useful for unsprung mass components. Low heat rejection engines have the potential for increased efficiency. Increased efficiency requires higher operating temperatures, which, in turn, requires materials that can tolerate such temperatures, namely, ceramics.
In a ceramic gas turbine engine, the ceramic parts are the combustor, nose cone, stators, rotors, rotor shrouds, and regenerators. Hot gas from the combustor is directed by the nose cone and stators against the blades of the rotors. The exhaust passes through the regenerators, which recover much of the residual heat, passing it on to the inlet air heading for the combustor. Of these ceramic components, the most demanding is the turbine rotor, which is subject to high mechanical stress, high temperature, thermal shock, and an oxidizing atmosphere.
Silicon nitride (Si.sub.3 N.sub.4) and silicon carbide (SiC) are useful for many applications. These materials form essentially covalent, three-dimensional structures and are extremely refractory. Because of their refractory nature, they are generally formed into solid bodies from powder by sintering, with or without external pressure. However, because of the high strength of their chemical bonds, atomic diffusion coefficients are extremely low below their respective decomposition temperatures (.about.1900.degree. C. for Si.sub.3 N.sub.4 and .about.3000.degree. C. for SiC), and the pure materials don't sinter well, even under pressure (hot pressing). To achieve dense Si.sub.3 N.sub.4 or SiC bodies, additives called sintering aids must be mixed with the powders before firing. Alternatively, a somewhat indirect sintering process called reaction bonding can be used. Hot pressing, incidentally, is unsatisfactory for preparing ceramic bodies of complex shape. Simple shapes can be machined into complex ones, but the cost is prohibitive.
Si.sub.3 N.sub.4 exists in two hexagonal crystalline forms that have similar atomic arrangements; the .beta. form, however, is slightly more symmetric than the .alpha.. The relationship between the two forms is not entirely clear, because the .alpha. readily transforms into the .beta. form on heating, but the reverse transformation has not been reported. Because silicon oxide (SiO.sub.2) is more stable than Si.sub.3 N.sub.4, the nitride owes its oxidation resistance to a passive oxide surface layer. A number of reactions can be used to prepare Si.sub.3 N.sub.4, among them the direct nitriding of silicon EQU 3Si+2N.sub.2 .revreaction.Si.sub.3 N.sub.4
the reaction of silicon tetrachloride with ammonia EQU 3SiCl.sub.4 +16NH.sub.3 .revreaction.Si.sub.3 N.sub.4 +12 NH.sub.4 Cl,
and the reaction of silica, carbon, and nitrogen EQU 3SiO.sub.2 +6C+2N.sub.2 .revreaction.Si.sub.3 N.sub.4 +6CO.
Other methods for the production of silicon nitride are illustrated in U.S. Pat. Nos. 4,073,845; 4,177,230; 4,264,565; 4,397,828; 4,399,115; 4,405,589.
A method for making Si.sub.3 N.sub.4 which is finding more industrial use is the use of organometallic halides to make polymeric silicon nitride precursors. However, one problem with silicon nitride made from organometallic precursors is its excessive residual halide content. Organometallic halides are nitrogen deficient and prone to excessive carbon content. Cross-linking and oligomer formation are associated with high carbon content and insoluble polymers. Organometallic halides often have volatile products and reduced yields. Control over product quality has been limited.
The following U.S. patents illustrate how to make polymeric precursors for silicon nitride: U.S. Pat. Nos. 3,809,713; 4,097,294.; 4,395,460; and 4,612,383.
U.S. Pat. No. 4,676,966 illustrates the making of silicon carbide from organosilicon compounds. The article appearing in the British Polymer Journal, Vol. 18, pgs. 355-358 (1986) discloses polymeric routes to silicon carbide.
The most commonly-used sintering aids for Si.sub.3 N.sub.4 are magnesium oxide (MgO) and yttrium oxide (Y.sub.2 O.sub.3). These additives apparently act by forming relatively low melting silicates with the passive silica layer on each nitride particle, and with any impurity oxides present. At sintering, or at hot pressing temperatures (.about.700.degree. C.), this silicate is liquid and promotes liquid-phase sintering. On cooling, it forms a glassy intergranular layer that binds the Si.sub.3 N.sub.4 grains together. This makes for an extremely strong material at room temperature, with flexural strengths over 100,000 psi. Unfortunately, at temperatures above about 1000.degree. C., the glassy intergranular layer softens, leading to a substantial loss of strength and resistance to creep. In this respect, Y.sub.2 O.sub.3 appears to be a better sintering aid than MgO, because strength is retained to a higher temperature. However, a different problem has been encountered with some Y.sub.2 O.sub.3 -sintered and hot-pressed silicon nitrides, namely, intermediate temperature oxidation. At around 1000.degree. C., although not at higher or lower temperatures, some of these materials appear to be quite prone to oxidation. Recent evidence suggests that this susceptibility to intermediate temperature oxidation is characteristic of certain intergranular phase compositions and that the problem might be minimized by suitable composition control.
The technique of preparing reaction-bonded Si.sub.3 N.sub.4 bodies involves forming the desired shape from silicon powder and then nitriding it. By choosing the proper density for the silicon powder body, it is possible, in principle, to obtain a fully dense Si.sub.3 N.sub.4 body with the same size and shape as the original silicon powder body, the Si.sub.3 N.sub.4 formed just filling the pores. However, in practice the interiors of bodies with densities greater than about 85% of theoretical cannot be fully nitrided because a dense Si.sub.3 N.sub.4 outer layer seals off the interior from access to nitrogen. Because the optimum nitriding temperature (1400.degree. C.) is close to the melting point of silicon, and because the nitriding reaction is exothermic, great care is exercised to avoid the formation of molten blobs of silicon in the interior that, because they are larger than the solid silicon particles, do not nitride completely and are sources of mechanical weakness in the product when they solidify. A relatively elaborate nitriding cycle, taking about a week, is employed. The nitriding rate is increased if a little iron is mixed with the silicon powder compact. The iron appears to break up or modify the passive oxide layer on the Si particles so that nitriding can proceed more easily.
Because reaction-bonded Si.sub.3 N.sub.4 (RBSN) is not fully dense and is about 15% pores, it is not as strong or oxidation resistant as hot-pressed material. On the other hand, because it lacks sintering aids, it retains its strength well at elevated temperature.
A recently developed variant of RBSN is sintered RBSN. This material is prepared by first making RBSN with sintering aid (e.g. Y.sub.2 O.sub.3) included, and then firing it to bring about further sintering. Densities near theoretical are achieved, with mechanical properties comparable to those of hot-pressed silicon nitride. The advantages of this technique are the ability to form complex shapes and the reduced shrinkage during firing (because one starts with RBSN), which, in turn, yields greater consistency in final size and shape.
SiC, the other high-performance ceramic seriously considered for highly-stressed gas turbine applications, exists in cubic (.beta.) and hexagonal (.alpha.) close-packed versions. The .beta. form is unique, but the .alpha. form has many variants, called polytypes, which involve long period changes in the layer-stacking sequence. Most SiC powder is still made by the Acheson process, essentially the reaction of SiO.sub.2 with carbon, with post-treatment to make it suitable for sintering, although other syntheses are also used.
As is the case with Si.sub.3 N.sub.4, SiC bodies can be prepared by sintering, hot pressing, and a form of reaction bonding. It is just as difficult to sinter or hot press pure SiC as it is to sinter or hot press pure Si.sub.3 N.sub.4. Again, sintering aids are used, but in the case of SiC different aids and higher temperatures are required. For hot pressing, the usual additive is aluminum oxide (Al.sub.2 O.sub.3), which may form a liquid with the passive oxide layer and provide a medium for liquid-phase sintering. As with hot-pressed Si.sub.3 N.sub.4, there is evidence of a decrease in strength at high temperature suggesting the presence of a glassy grain boundary phase. Sintering of SiC is usually carried out at 2000.degree. C. or above, using carbon and boron as sintering aids. It is believed that the carbon removes the passive silica layer, whereas the boron enters the SiC grains, modifying their surface energy to enhance sintering. Liquid-phase sintering does not appear to be involved, and strength is retained to very high temperature.
Reaction-bonded SiC bodies are made by forming the desired shape from SiC powder, an organic binder, and, in some cases, carbon powder. This body is carbonized to convert the organic binder to carbon and then silicided in liquid silicon to convert the carbon to SiC. The resulting bodies can be quite strong but always contain residual silicon which causes them to weaken at about 1300.degree. C. The increase in strength with temperature up to about 1200.degree. C. is believed to result from the plastic flow of silicon serving to heal cracks.
As mentioned above, polymeric precursors to ceramics have been widely investigated. Polymeric precursors are conducive to part shape flexibility, control of chemistry on a molecular level, and high purity starting materials. Polymers can be used with ceramic powders as binders to improve density, provide green strength, and/or supply sintering aids uniformly. The density of reaction bonded silicon nitride (RBSN), usually about 70%, can be increased by infiltrating polymeric silicon nitride precursors. Ceramic fibers such as NI CALON or NEXTEL are made from polymeric precursors. Polymer ceramic precursors, also referred to as sol-gels, can be used to make monoliths. Silicon nitride made from low-cost precursors have enjoyed limited success in the past due to residual constituents. The usefulness of more complex precursors to silicon nitride is limited by extremely high cost.